1. Field of the Invention
This invention relates to regulated, direct current (dc) voltage supplies. More particularly, this invention relates to solid-state, band-gap voltage references which are capable of maintaining a dc output voltage having a substantially constant magnitude even while being subjected to power supply voltage variations and which require compensation capacitances of minimum size.
2. Description of the Prior Art
U.S. Pat. No. 3,887,863 ('863) of Adrian Paul Brokaw, discloses a two-transistor band-gap voltage reference source or cell wherein the ratio of current densities of the two transistors is automatically controlled to a predetermined magnitude by a negative feedback amplifier. A voltage having a positive temperature coefficient (TC) corresponding to the difference in base-to-emitter voltages (.DELTA.V.sub.BE) of the two transistors is developed and connected in series with the base-to-emitter voltage (V.sub.BE) of one of the two transistors having a negative TC to form a composite voltage at the base of the transistor. The circuit parameters can be selected so that a composite reference voltage having a magnitude near the band-gap voltage of silicon and a low or minimal temperature coefficient is provided.
If output voltages having greater magnitudes are desired then a voltage divider network comprising two series-connected resistors can be connected to the output terminal of the negative feedback amplifier. A common junction between these resistors is connected to the commonly connected bases of the two reference transistors. The divider provides a reference voltage which is a predetermined fraction of the output voltage to the junction. Therefore, the magnitude of the resulting output voltage at the output of the negative feedback amplifier can be a predetermined multiple of the reference or bandgap voltage.
The negative feedback path of FIG. 4 of the foregoing '863 Brokaw patent includes a PNP current mirror which converts the differential output signals of the two transistor voltage reference source into a single-ended signal. The PNP current mirror is coupled through a voltage level shifting stage to an NPN Darlington output driver stage which is coupled to the aforementioned series-connected resistors. More specifically, the current mirror includes three PNP transistors. The first PNP transistor has a base electrode connected to the collector electrode of one of the two transistors of the voltage reference source. The collector electrode of the first mirror transistor is connected to the negative supply conductor. A second of the PNP current mirror transistors has a collector electrode connected to the base electrode of the first current mirror transistor and to the collector electrode of the one voltage reference source transistor. The third PNP current mirror transistor has a collector electrode connected to the collector electrode of the other voltage reference transistor, and a base electrode connected to the base electrode of the second current mirror transistor and to the emitter of the first current mirror transistor. The emitter electrodes of the second and third current mirror transistors are each electrically connected to a common conductor which is coupled through a power supply voltage pre-regulator circuit to the positive d.c. power supply line.
During operation, assume that the collector voltage of the first reference source transistor drives the base of the first current mirror transistor toward the negative level. The first current mirror transistor, acting as an emitter-follower, then renders the other two current mirror transistors conductive until the magnitude of their collector currents approximately equals the magnitude of the collector current of the first reference source transistor. Since the base-to-emitter junctions of the second and third current mirror transistors are connected in parallel, the collector current of the third current mirror transistor is approximately equal to or "mirrors" the collector currents of the second current mirror transistor and the first reference source transistor. This desirable condition exists only so long as the voltage between the negative power supply conductor and the common conductor has a constant magnitude. Since the d.c. power supply voltage varies for many reasons, such as ripple for instance, usage of the aforementioned power supply pre-regulator circuitry is required which adds complexity and reduces yields thereby undesirably increasing the cost of the integrated circuit. If the separate power supply voltage regulator is not employed then problems result from the different base width modulations of the transistors of the reference cell in response to power supply voltage variation causing current imbalance therein as pointed out in column 6, line 60 of the foregoing '863 Brokaw patent.
The foregoing problem is also discussed hereinafter wih respect to FIG. 1 hereof which relates to another "modified" version of the Brokaw circuit and which is prior art to the subject invention.
The process and geometry utilized in manufacturing the three PNP mirror transistors commonly used in such bipolar integrated circuits results in such devices having poor frequency responses and low gains. Thus, the PNP current mirror utilized in the above-described feedback path tends to have a poor frequency response which results in phase and stability problems, thereby requiring a frequency compensation capacitance having a high value and thereby taking up an undesirable amount of die area. The frequency compensation capacitor is connected between the collector electrode of the third current mirror transistor and the negative supply conductor. Also, the large capacitor undesirably slows down the regulator reaction time in response to transients.
3. Objects of the Invention
It is therefore an object of the present invention to provide general purpose voltage reference sources having relatively simple configurations as compared to prior art circuits.
Another object of the invention is to provide simple general purpose voltage reference sources having a high degree of rejection to the variation of the magnitude of the power supply line voltages.
Still a further object of the invention is to provide a band-gap voltage reference source which requires a frequency compensation capacitor of minimum size.